The doctors will believe you. Depression is something you can see in a person's eyes - so don't worry because they will see it and will give you appropriate treatment for it. Just go as soon as possible because I can only see good things coming out of that.
Hang in there Sphinxy - I've been there myself, I even tried to kill myself once, but now I just look back at it and think how ridiculous it all was. Take my words and trust me - I know it's hard to accept any truths at this stage for you but this is one truth you must accept.
The thing about depression is that it comes and goes. Right now it came, but it will go. Don't give in to depression though because it is hard to pull out of it but everyone can pull themselves out of it.
Like I said, see a doctor as soon as possible. After you have seen the doc - talk to your parents, your friends, counselor, pastor/minister, and whoever else you know that you can talk to. Just tell them exactly how you feel and they will do everything they can to make sure you are feeling great again.